In the prior art, to form an integrated circuit on a semiconductor substrate, a resist film on which is formed a circuit pattern is arranged on the surface of a semiconductor substrate. Then, layers under the resist film, such as an insulation film, a semiconductor film, or a metal film, are etched through the resist film. The resist film is removed from the substrate surface after ending the etching process. One example of a method for removing the resist film is a dry processing method for ashing (incinerating) the resist film using the plasma of reactive gas, mainly oxygen plasma.
The dry processing method causes reaction of active species (radicals), mainly oxygen radicals, generated in the plasma of the reactive gas, in the resist film applied to the substrate, to decompose and vaporize the resist film to CO2 and H2O for removal. Patent document 1 discloses an example of a plasma ashing device for removing a resist film through the dry processing method. This ashing device will be described with reference to FIG. 7.
As shown in FIG. 7, an ashing device includes a chamber (processing chamber) 1, the upper part of which is coupled to a feed tube 2. The feed tube 2 is connected to a plasma chamber (not shown) which generates plasma. A shower plate 3, which includes a plurality of through holes, is arranged at the lower end of the feed tube 2 facing toward a substrate stage 4. A cylindrical diffusion prevention wall 5 is attached to an upper inner surface of the processing chamber 1 so as to extend around the shower plate 3. A high frequency power supply 6 is connected to the substrate stage 4. A ventilation port 7 is formed at the bottom of the chamber 1.
The ashing process performed by the ashing device of FIG. 7 will now be described. First, a substrate (wafer) W arranged in the chamber 1 is mounted on an upper surface of the substrate stage 4. The interior of the chamber 1 is depressurized, and high frequency voltage is applied to the substrate stage 4. Then, gas containing oxygen radicals is supplied to the chamber 1 through the feed tube 2. The gas containing oxygen radicals flows through the through holes of the shower plate 3 and reaches the substrate W. The gas flowing outward from the shower plate 3 is guided by the diffusion prevention wall 5 towards the substrate W. A resist film (not shown) formed on the upper surface of the substrate W is decomposed and vaporized by the oxygen radicals contained in the gas and then discharged from the ventilation port 7.
In the integrated circuit on the semiconductor substrate, circuit elements such as transistors are connected by a metal wiring of aluminum (Al), copper (Cu), or the like. Some integrated circuits have connection pads of which surfaces are covered by gold (Au) or the like or connection terminals formed from solder. Thus, when manufacturing the semiconductor substrate, during the ashing of the resist film, the metal wiring may be exposed and gold or solder may be formed on the surface. In such a case, the exposed metal material is sputtered by chemical reactions or physical reactions. This scatters metal atoms, and the metal atoms collect on the inner walls of the chamber 1, that is, the lower surface of the shower plate 3 and the inner circumferential surface of the diffusion prevention wall 5. If the ashing process is continued in such a state, the metals collected on the inner walls of the chamber 1 bond with the oxygen radicals that should be guided to the substrate W. This oxidizes the metal surface and increases the amount of deactivated oxygen radicals. In other words, the metal collected on the inner wall of the chamber 1 increases the amount of deactivated oxygen radicals. As a result, the amount of oxygen radicals that reaches the substrate W decreases, and the depth (ashing rate) of the resist film that can be processed during the same time decreases. Furthermore, the metal atoms scattered from the substrate W are collected on the inner walls of the chamber 1 in a non-uniform manner. This lowers the uniformity of the ashing rate in the surface of the substrate W. The inventors of the present invention have confirmed that the metals scattered from the substrate W decrease the ashing rate and lowers the in-surface uniformity through experimental results, which are described below.
FIGS. 9 and 10 are graphs showing the measurement values of the ashing depth in the substrate W. Referring to FIG. 8, the measurement values indicate the ashing depths from the surface of the resist film at forty-nine measurement points on the substrate W, which are set in order from the center of the substrate W in the circumferential direction and the radial direction. In FIGS. 9 and 10, the black circles represent the measurement values taken when performing the ashing process after the chamber 1, the shower plate 3, and the diffusion prevention wall 5 are all washed. The black squares represent the measurement values taken when performing the ashing process again using the used shower plate 3 and diffusion prevention wall 5. The black triangles represent the difference between the measurement value represented by the black circles and the measurement values represented by the black squares.
FIG. 9(a) is a graph showing the measurement results of when a used shower plate 3 and diffusion prevention wall 5, which were used during a previous ashing are set in a new chamber 1, and re-ashing is performed on the substrate W from which copper is exposed under a first ashing condition (processing condition A). FIG. 9(b) is a graph showing the measurement result of when the same process as FIG. 9(a) is performed under a second ashing condition (processing condition B), which differs from the first ashing condition. FIG. 10(a) is a graph showing the measurement results of when a used diffusion prevention wall 5, which were used during a previous ashing are set in a new chamber 1, and re-ashing is performed on the substrate W from which gold is exposed under processing condition A. FIG. 10(b) is a graph showing the measurement result when the same process as FIG. 10(a) is performed under processing condition B. The processing time is the same for each case (30 seconds).
As apparent from FIGS. 9 and 10, when the shower plate 3 and the diffusion prevention wall 5 of the ashing device that have processed a substrate, from which metal (copper, gold) was exposed, are set in a chamber 1, which has been washed, and the ashing process is performed (refer to black squares), the ashing depths all decrease compared to when the ashing process is performed in the ashing device in which the chamber 1, the shower plate 3, and the diffusion prevention wall 5 are all washed (refer to black circles). In particular, in FIG. 9(a), the ashing depths of the measurement points 1 to 9 and the measurement points 26 to 49 under the condition represented by the black squares are significantly decreased, and in FIG. 10, the ashing depths of the measurement points 26 to 49 under the condition represented by the black squares decrease significantly. In the case of the condition represented by the black squares, a large amount of the oxygen radicals that should reach the measurement points 1 to 9 and 26 to 49 are supplied toward the measurement points via the shower plate 3 or the diffusion prevention wall 5 on which metals are collected. It is thus assumed that the metals collected on the shower plate 3 and the diffusion prevention wall 5 deactivate a large amount of oxygen radicals thereby significantly decreasing the amount of oxygen radicals that reach the measurement points 1 to 9 and 26 to 49 and significantly decreasing the ashing depth at such measurement points.
This also shows that the amount of metal collected in the path of the oxygen radicals (shower plate 3, diffusion prevention wall 5, etc.) varies the amount of oxygen radicals that reach each measurement point. This, in turn, varies the ashing depth at each measurement point. Actually, as apparent from the results shown by the black squares in FIGS. 9 and 10, the ashing depth varies in the surface of the substrate W when the metal distribution state on surfaces facing toward the substrate W is non-uniform, such as when metals are not collected in the chamber 1 but collected on the shower plate 3 and the diffusion prevention wall 5.    Patent Document 1: Japanese Laid-Open Patent Publication No. 9-45495